SDH/SONET transmission equipment is used to transport streams of data in a transmission network. Transport is segmented into frames, which are time segments of transmission, lasting typically 125 microseconds. The SDH/SONET equipment inserts header data, e.g., section overhead (SOH) and so on, as well as payload information in each frame. The payload information in a frame contains data from the stream that is transported, so that payload data from successive frames makes up a continuous stream. By definition, the header data is data that is generated per frame. By way of example, header data includes information such as synchronization data for locating the start of a frame, error detection data for the frame, communication channels between equipment for transporting information such as management and protection coordination information, frame source identification information, national bytes, and so on.
In principle, frames generated at different nodes are asynchronous. That is, their duration may differ slightly and they have no fixed phase relation. Thus, a one-to-one relation does not exist between frames transported in the network or even along a connection via various nodes through the network. Even when two nodes initially start frames at substantially the same time, the start time of a frame having a number n (e.g., a large integer number) of frames after the initial frame in one of the nodes can differ by more than a frame period from the starting time of the frame started n frames after the initial frames in the other node.
This is not a problem for header data because, when the data stream is transported along the connection, nodes generate new headers for their own outgoing frames. Payload data, on the other hand, has to be copied from incoming frames and no data may be lost. Therefore, the nodes have the ability to float the payload data relative to the frames. That is, data starting from a beginning of the payload of an incoming frame does not have to start at a predetermined position in the outgoing frame and the data of the payload in an incoming frame may be distributed over the payloads of successive outgoing frames.
As is well known, header data may be used at a node to manage the transmission network. In principle, a network operator can choose to use any of the bytes from the header in the nodes of the network to manage the network in any particular manner. In practice, however, a network operator may not be able to do this because of the way in which payload data is transported through the network, e.g., if the payload data is transported from a first sub-network of a first operator to a second sub-network of that first operator via a third sub-network of a second operator. Because there need not be a one-to-one correspondence between headers coming out of the last node of the first sub-network (before entering the third sub-network) and headers entering the second sub-network (from the third sub-network), the third sub-network is not transparent to the first operator. Information that is relevant to the first network operator may be lost when the nodes of the third sub-network generate headers as desired by the second operator. The extent to which there is freedom to copy header data from one header to another is often severely limited. For example, consider the situation where a first and second SDH sub-network that carry frames comprising STM-16 signals are connected via an SDH sub-network that carries frames with STM-64 or STM-256 signals. An STM-16 signal has 1152 header bytes, but only about 25 of these bytes are forwarded in the STM-64 or STM-256 signal.
Accordingly, this can obstruct various network management operations of the first network operator. Also timing information, e.g., about the bit rate of the frames, is lost upon passage through the third sub-network. This can also obstruct various network management operations of the first network operator.
A known solution to this problem is to require the third sub-network to use signals with frames of a larger bandwidth, which allows both the header data from the first sub-network and header data generated for the third sub-network to be transported. However, this requires significant overhead, which is typically not feasible or desirable in SDH/SONET-based transmission.